The long term goal of this research is to understand the neural mechanisms that regulate jaw movements. There is a poor understanding of the role of the major neuronal components (motor cortex, cerebellum, and basal ganglia) in the control of modulation of voluntary and rhythmic jaw movements. The proposed project will examine the effects of cerebellar and basal ganglia (i.e., Parkinson's disease) dysfunction in man on the timing, force production, and sequencing of muscle activity during voluntary and rhythmic jaw movements. Two specific aims are proposed: (1) To determine what changes in jaw movement occur in subjects with cerebellar-disease and (2) to determine what changes in jaw movement occur in subjects with Parkinson's disease. The performance of the patients will be compared with age-matched control subjects. It is hypothesized that the subjects with cerebellar dysfunction will show deficits in voluntary jaw movements when timing is critical, but no deficits in normal chewing. It is anticipated that subjects with Parkinson's disease will show performance deficits in tasks requiring speed and force production but no deficits in normal opening and closing of the mouth, normal chewing, or slowed rhythmic jaw movements. The three-dimensional movements of the mandible will be recorded, which will provide data on the position, velocity, and trajectory of the jaw during all tasks. Electromyographic (EMG) activity will be recorded from the right and left temporalis, masseter and digastric muscles. A miniature bite force sensor will measure force during jaw closing. After a brief oral examination, the subjects will be tested on the following tasks: normal and fast opening, closing, and lateral movements of the jaw; normal chewing and paced rhythmic jaw movements; and short and prolonged clenching. Results from the proposed studies will elucidate the effects of cerebellar dysfunction or Parkinson's disease on jaw movements. It is anticipated that this study will serve as the basis of subsequent work on the mechanisms of cerebellar and basal ganglia control of the jaw and will be useful in developing appropriate dental treatment plans for patients with these severe motor disorders (e.g., employing dental appliances that minimize particular force vectors).